1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus which purifies exhaust gases exhausted from an internal combustion engine, and temporarily adsorbs hydrocarbons within exhaust gases upon start of the internal combustion engine.
2. Description of the Prior Art
The type of conventional exhaust gas purifying apparatus for an internal combustion engine mentioned above is known, for example, from Laid-open Japanese Patent Application No. 2000-310113. The exhaust gas purifying apparatus disclosed therein comprises a pair of upstream and downstream three-way catalysts in an exhaust pipe of an internal combustion engine. An inner main exhaust passage and an annular bypass passage around the main exhaust passage are formed between the two three-way catalysts within the exhaust pipe. The bypass passage has a passage area smaller than the main exhaust passage, and is filled with a hydrocarbon adsorbent. A switching valve is also provided at the inlet port of the main exhaust passage for opening and closing the main exhaust passage.
For controlling the switching valve, it is determined whether or not the following three conditions are met after the engine is started:
1) whether or not a cooling water temperature of the engine detected by a water temperature sensor is lower than a predetermined temperature;
2) whether or not the amount of intake air detected by an air flow meter is smaller than a predetermined amount; and
3) whether or not a time elapsed after the start is shorter than a catalyst activation time which is determined in accordance with the cooling water temperature.
When the three conditions are all met, the switching valve is fully closed on the assumption that the downstream three-way catalyst has not been activated. In this state, exhaust gases passing through the upstream three-way catalyst are entirely passed to the bypass passage, so that hydrocarbons within the exhaust gases are adsorbed by the adsorbent filled in the bypass passage. Then, the exhaust gases flow into the downstream three-way catalyst, thereby preventing hydrocarbons from being emitted to the atmosphere. On the other hand, when any of the three conditions is not met, the switching valve is fully opened on the assumption that the downstream three-way catalyst has been activated. In this state, a madownstream three-way catalyst for purification through its oxidation/reduction actions.
However, the conventional exhaust gas purifying apparatus determines whether the downstream three-way catalyst is activated after the start of the engine based on the cooling water temperature, amount of absorbed air, and time elapsed after the start, which are used as parameters, so that the exhaust gas purifying apparatus may fail to make appropriate determination, resulting in the inability to switch the switching valve at an appropriate timing. For example, since the downstream three-way catalyst is located substantially away from the engine body for which the cooling water temperature is detected, the temperature of the downstream three-way catalyst rises with a delay from the cooling water temperature. As such, the cooling water temperature does not always match the temperature of the downstream three-way catalyst in rising timing, behavior and the like. Therefore, the cooling water temperature may not exactly reflect the actual temperature state of the downstream three-way catalyst, i.e., whether it is activated.
To solve the disadvantage as mentioned above, the temperature of the downstream three-way catalyst may be directly detected by a temperature sensor for use as a parameter instead of the cooling water temperature. With this strategy, however, the activation of the three-way catalyst cannot either detected with high accuracy because the temperature sensor generally has a responsibility too low for use with the downstream three-way catalyst which is activated in a relatively short time after the start of the engine, and also because the temperature sensor experiences difficulties in detecting the temperature at the center of the downstream three-way catalyst, which is critical for evaluating whether or not the three-way catalyst is activated, in a temperature distribution of the three-way catalyst which can readily vary when the temperature rises in such a short time.
Also, since the temperature rising rate of the downstream three-way catalyst depends on a particular operating condition after the start of the engine (for example, when the vehicle is idled after the start, and when the vehicle is launched immediately after the start), the time elapsed after the start does not either reflect exactly an actual activated state of the downstream three-way catalyst. Further, in regard to the amount of intake air, since a detection value detected every predetermined time is compared with a predetermined value, the conventional exhaust gas purifying apparatus will erroneously determine that the downstream three-way catalyst has been activated if the amount of intake air instantaneously increases. From the result of the foregoing analysis, the conventional exhaust gas purifying apparatus cannot set a timing at which the switching valve is switched to the main exhaust passage appropriately in response to a transition of the downstream three-way catalyst into activation. Consequently, if the switching valve is switched at a timing too early, exhaust gases will flow into the inactivated downstream three-way catalyst, so that hydrocarbons will be emitted to the atmosphere to exacerbate the exhaust gas characteristic. On the other hand, if the switching valve is switched at a timing too late, exhaust gases will flow into the downstream three-way catalyst with a delay, though it has been already activated, thereby failing to effectively utilize the purifying performance.
Also, a substantial amount of intake air reduced in a highland environment, as compared with a flatland environment, results in a lower combustion temperature which fails to sufficiently raise the temperature of exhaust gases, causing a delay in the activation of the three-way catalyst and a lower rate at which the adsorbent is heated. For this reason, a longer time is required for the adsorbent to be heated to a temperature at which hydrocarbons can be desorbed therefrom in the highland environment than in the flatland environment. The prior art, however, does not take into account the temperature rise characteristics of the three-way catalyst or adsorbent in such a highland environment, so that exhaust gases flowing into the inactivated downstream three-way catalyst will exacerbate the exhaust gas characteristics. In addition, since the switching valve is switched to the main exhaust passage before the desorption temperature is reached, the desorption actually starts with a delay, resulting in a longer time required to completely desorb hydrocarbons from the adsorbent. With such a delay in completing the desorption, if the engine is stopped immediately after it has been started, for example, hydrocarbons, which are not desorbed, remain within the adsorbent, so that the adsorbent fails to sufficiently demonstrate the adsorption performance at the time the engine is next started.